Watch these plants "sleep" and "wake up" over a 24-hour period. The phenomenon is called nyctinasty which describes the movement of plants in response to changes in light and temperature over the circadian cycle. From Biology Online:
Nyctinasty is different from a tropic movement (tropism). Tropism is a directional movement or growth response of a cell or an organism to a stimulus, which may either be positive or negative depending on the source and kind of stimulation. In other words, tropism is associated with the growth response of the plant to a stimulus. Phototropism, for instance, is a form of tropism wherein plants grow towards the source of light (positive tropism). In nyctinasty, some plants are able to assume a position at night that is different from their position during daytime. It is a biological rhythm since this behavior recurs in each circadian day. The sleeping position of these plants are said to be associated with pulvinar movement, circadian clock, and light signal transduction through phytochrome. Pulvinar movement involves pulvinar cells at the base of a plant leaf (or leaflet) or at the apex of the petiole that facilitates nyctinastic and nastic movements in a mechanism similar to stomatal closure.
